1. Field of the Invention
This invention relates to an improvement in the head part of a power operated screw driver, and more particularly, to a head having holding jaws for holding screws, sent one by one through a pipe and having stop jaws for preventing the return of the screws.
2. Description of the Prior Art
In the past, for example, in the U.S. Pat. No. 3,247,874, two holding jaws were provided freely pivoted across the end of a head part in such a manner that they approached each other, and one stop jaw was provided freely pivoted at right angles to the pivoting directions of the holding jaws. Therefore, with such a construction, however, it is impossible to drive screws into parts having restricted widths or into deep recesses. Another disadvantage of such a construction is the lack of screw stability because a screw head being held by the holding jaws is retained at one position by one stop jaw.
Japanese Utility Model Journal Publication No. 44-18318 discloses a screwdriver in which stop jaws are built in to pivot inside each of a pair of opposing holding jaws and springs are attached to the backs of the stop jaws so that the stop jaws are biased to approach each other. This mechanism has solved the drawbacks discussed above with regard to U.S. Pat. No. 3,247,874. However, since the stop jaws are built in, the width of the holding jaws in the pivoting direction was increased. That is, the presence of leaf springs at the back of stop jaws and the wall thickness of the holding jaws proper contributes to a corresponding increase in the width of the holding jaws. Further, since the two holding jaws are set across the tube which guides the bit, the width of the entire head is widened at least twice as large as the increment. This causes an extremely important problem considering that this kind of screwdriver is often used in restricted areas and that even a slight increase in the head part width may greatly confine the application range. Additionally, in the event of replacement of the stop jaws or spring when the screw head shape is changed or if the spring for the stop jaw is broken, the replacement work takes a great deal of time and is very complicated because the entire holding jaw must be disassembled. Another problem is the risk of screwdriver destruction by continuing a screwing operation after breakage of the stop jaw spring, since such breakage is not visible from the outside.